Lossless microwave switch based on tunable filters for quantum information processing

ABSTRACT

A technique relates to a lossless microwave switch discussed herein. Multiple ports are included in the lossless microwave switch. Tunable filters are included in the lossless microwave switch. Each of the ports is operatively coupled a corresponding one of the tunable filters.

BACKGROUND

The present invention relates to superconducting electronic devices, andmore specifically, lossless microwave switches and/or routers based ontunable filters for quantum information processing.

A radio frequency (RF) and microwave switch is a device to route highfrequency signals through transmission paths. RF and microwave switchesare used extensively in microwave test systems for signal routingbetween instruments and devices. Incorporating a switch into a switchmatrix system enables one to route signals from multiple instruments tosingle or multiple devices. Similar to electrical switches, RF andmicrowave switches come in different configurations providing theflexibility to create complex matrixes and automated test systems formany different applications.

In physics and computer science, quantum information is information thatis held in the state of a quantum system. Quantum information is thebasic entity of study in quantum information theory, and can bemanipulated using engineering techniques known as quantum informationprocessing. Much like classical information can be processed withdigital computers, transmitted from place to place, manipulated withalgorithms, and analyzed with the mathematics of computer science,analogous concepts apply to quantum information. Quantum systems such assuperconducting qubits are very sensitive to electromagnetic noise, inparticular in the microwave and infrared domains.

SUMMARY

According to one or more embodiments, a lossless microwave switch isprovided. The lossless microwave switch includes a plurality of portsand tunable filters. Each of the plurality of ports is operativelycoupled to a corresponding one of the tunable filters.

According to one or more embodiments, a method of configuring a losslessmicrowave switch is provided. The method includes providing a pluralityof ports and providing tunable filters. Each of the plurality of portsis operatively coupled to a corresponding one of the tunable filters.

According to one or more embodiments, a lossless microwave switch isprovided. The lossless microwave switch includes a plurality of portsand tunable filters. Each of the plurality of ports is associated withone of the tunable filters. Each of the tunable filters includes asuperconducting quantum interference device.

According to one or more embodiments, a lossless microwave switch isprovided. The lossless microwave switch includes a node and tunablefilters connected to the node. The tunable filters are configured to beindependently tuned to a first state to transmit a signal and beindependently tuned to a second state to block the signal, such that anyone of the tunable filters is configured to transmit the signal to anyother one of the tunable filters via the node.

According to one or more embodiments, a lossless microwave switch isprovided. The lossless microwave switch includes a plurality of ports. Afirst pair of the plurality of ports has a tunable filter connected inbetween, in which the tunable filter is configured to transmit amicrowave signal. A second pair of the plurality of ports has anothertunable filter connected in between, in which the other tunable filteris configured to reflect the microwave signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a superconducting microwave switch/routeraccording to one or more embodiments.

FIG. 2 is a block diagram of the superconducting microwave switch/routerin FIG. 1 according to one or more embodiments.

FIG. 3 is a schematic of the superconducting microwave switch/routerillustrating transmission as the mode of operation according to one ormore embodiments.

FIG. 4 is a schematic of the superconducting microwave switch/routerillustrating reflection as the mode of operation according to one ormore embodiments.

FIG. 5 is a schematic of a superconducting microwave switch/routeraccording to one or more embodiments.

FIG. 6 is a block diagram of the superconducting microwave switch/routerin FIG. 5 according to one or more embodiments.

FIG. 7 is a schematic of a superconducting microwave switch/routeraccording to one or more embodiments.

FIG. 8 is a schematic of a superconducting microwave switch/routeraccording to one or more embodiments.

FIG. 9 is a schematic of a N port superconducting microwaveswitch/router according to one or more embodiments.

FIG. 10 is a flow chart of a method of configuring a superconductingmicrowave switch/router according to one or more embodiments.

FIG. 11 is a flow chart of a method for configuring a superconductingmicrowave switch/router according to one or more embodiments.

FIG. 12 is a flow chart of a method of configuring a superconductingmicrowave switch/router according to one or more embodiments.

FIG. 13 is a flow chart of a method of configuring a superconductingmicrowave switch/router according to one or more embodiments.

DETAILED DESCRIPTION

Various embodiments are described herein with reference to the relateddrawings. Alternative embodiments can be devised without departing fromthe scope of this document. It is noted that various connections andpositional relationships (e.g., over, below, adjacent, etc.) are setforth between elements in the following description and in the drawings.These connections and/or positional relationships, unless specifiedotherwise, can be direct or indirect, and are not intended to belimiting in this respect. Accordingly, a coupling of entities can referto either a direct or an indirect coupling, and a positionalrelationship between entities can be a direct or indirect positionalrelationship. As an example of an indirect positional relationship,references to forming layer “A” over layer “B” include situations inwhich one or more intermediate layers (e.g., layer “C”) is between layer“A” and layer “B” as long as the relevant characteristics andfunctionalities of layer “A” and layer “B” are not substantially changedby the intermediate layer(s).

In accordance with one or more embodiments, superconducting (orlossless) microwave switches/routers allow one to route quantum signalson demand between different nodes of a circuit or between differentports. Superconducting microwave switches can have many applications inthe area of quantum information processing. For example, superconductingmicrowave switches can be utilized for time-multiplexed readout,time-multiplexed driving (e.g., cross-resonance drives),time-multiplexed characterization of several devices, time-multiplexedinteraction between pairs of quantum systems, time-dependent circulationof signals, etc.

According to one or more embodiments, a superconducting microwave switchthat can have one input port and N output ports is provided. Also, thesuperconducting microwave switch can have one output port and N inputports. Each of the ports of the superconducting microwave device isdesigned to have the same characteristic impedance Z₀. In oneimplementation, each input-output pair is connected through a tunablelow-pass filter whose cutoff frequency can be tuned in-situ usingapplied magnetic flux. The tunable low-pass filter can be implementedusing a ladder of series inductive elements (e.g., direct current (DC)superconducting quantum interference devices (SQUIDs)) and shuntcapacitive elements (e.g., lumped-element capacitors). In anotherimplementation, each input-output pair can be connected through atunable high-pass filter whose cutoff frequency can be tuned in-situusing applied magnetic flux, and the tunable high-pass filter can beimplemented using series capacitive elements (e.g., lumped-elementcapacitors) and shunt inductive elements (e.g., DC-SQUIDs).

Now turning to the figures, FIG. 1 is a schematic of a superconductingmicrowave switch/router 100 according to one or more embodiments. FIG. 1illustrates building blocks of the superconducting microwaveswitch/router 100 based on a tunable filter 20. In this example, thetunable filter 20 is a tunable low-pass filter (TLPF). However, itshould be appreciated that embodiments are not limited to low-passfilters as discussed further below.

In this example, the microwave switch/router 100 includes ports 10, suchas for example, ports 1 and 2. The ports 10 are input and output ports.The tunable filter 20 includes one or more unit cells 60. Each unit cell60 includes a variable inductor 40 designated as variable inductiveelement L₁ (and other examples include L2, L3, and DC-SQUIDs discussedfurther below), and each unit cell 60 includes a capacitor 50 designatedas capacitive element C. In each unit cell 60, the variable inductor L₁40 is connected in series with ports 10, and a capacitor C 50 isconnected to one end of the variable inductor 40 and to ground. Therecan be N number of unit cells 60 repeated and connected together (inseries) in the tunable filter 20 for a total of N unit cells. For N unitcells, the inductors L1 40 are connected in series, with each inductorL1 40 shunted to ground by its respective capacitor 50. Theinterconnection of the ports 10, variable inductors L1 40, andcapacitors C 50 is by transmission line 30. The transmission line 30acts as a superconducting wire or waveguide to carry a microwave signalfrom port 1 via the tunable filter 20 to port 2, or vice versa. Acoaxial cable can connect to the external ends of the ports 10 such thatone coaxial cable inputs microwave signals and another coaxial cableoutputs the microwave signals. The transmission line 30 can be astripline, microstrip, etc. The variable inductors 40, capacitors 50,and transmission lines 30 are made of superconducting material. Examplesof superconducting materials (at low temperatures, such as about 10-100millikelvin (mK), or about 4 K) include niobium, aluminum, tantalum,etc.

FIG. 2 is a block diagram of the superconducting microwave switch/router100 in FIG. 1 according to one or more embodiments. FIG. 2 is anequivalent circuit to FIG. 1 without depicting the internal details ofthe tunable filter 20.

It can be assumed that the microwave signal that is to be transmittedthrough the superconducting microwave switch/router 100 has a centerangular frequency ω₀. The impedance designation Z₀ is the characteristicimpedance at ports 1 and 2 (which can be the input and output ports orvice versa). For example, the characteristic impedance Z₀ can be 50 ohms(Ω) at each port.

For an individual unit cell 60, the impedance is Z₁ where

$Z_{1} = \sqrt{\frac{L_{1}}{C}}$and where the angular frequency ω₁ of the unit cell 60

$\omega_{1} = {\frac{1}{\sqrt{{CL}_{1}}}.}$The cutoff angular frequency of the tunable filter 20 denoted as ω_(C)is on the order of the angular resonance frequency ω₁ of the unit cell60 (or multiple unit cells added together) and it is correlated with ω₁,meaning ω_(C) increases and decreases with ω₁. The exact dependence ofω_(C) on ω₁ and on the number of unit cells N can be found through amicrowave simulation or calculation. From this it follows that thecutoff frequency ω_(C) of the tunable filter 20 depends on the values ofthe variable inductor L₁ 40 and the capacitor C 50 (for the one or moreunit cells 60). In particular, the inductance of the variable inductorL₁ 40 controls the cutoff frequency ω_(C) of the tunable filter 20,thereby controlling when the tunable filter 20 is operating intransmission or reflection with respect to the microwave signal (centerangular frequency ω₀). The inductance of the variable inductors L₁ 40has an inverse relationship to the cutoff frequency ω_(C). For example,when the inductance of the variable inductor L₁ 40 is increased, thecutoff frequency ω_(C) of the tunable filter 20 is decreased.Conversely, when the inductance of the variable inductor L₁ 40 isdecreased, the cutoff frequency ω_(C) of the tunable filter 20 isincreased. It is noted that varying the inductance of the unit cellsdoes not only change the cutoff frequency of the filter but also changesits characteristic impedance. Therefore, it can be desirable that Z₁ orthe characteristic impedance of the filter matches the characteristicimpedance of the ports as much as possible when the switch is closed,i.e., operated in the transmission mode.

Accordingly, when operating as a closed switch, the superconductingmicrowave switch/router 100 is controlled to pass the microwave signal(center angular frequency ω₀) in transmission from port 1 to port 2 (orvice versa) by decreasing the inductance of the variable inductor L₁ 40in the tunable filter 20. This allows the microwave signal (centerangular frequency ω₀) to fall within the low-pass band of the tunablefilter 20. When operating as an open switch, the superconductingmicrowave switch/router 100 is controlled to block transmission of themicrowave signal (center angular frequency ω₀) from port 1 to port 2 (orvice versa) using reflection by increasing the inductance of thevariable inductor L₁ 40 in the tunable filter 20. This allows themicrowave signal (center angular frequency ω₀) to fall outside of thelow-pass band and thus be attenuated or in other words reflected.

FIG. 3 is a schematic of the superconducting microwave switch/router 100illustrating transmission as the mode of operation according to one ormore embodiments. In FIG. 3, the tunable filter 20 is tuned such thatthe center angular frequency ω₀ of the incoming microwave signal 305through the device port is less than the cutoff frequency ω_(C) of thetunable filter 20, i.e., ω₀<ω_(C). In this mode of operation, thetunable filter 20 is configured to operate in transmission because thefrequency of the microwave signal 305 is less than the cutoff frequencyof the tunable low-pass filter 20. Under this condition, the microwavesignal 305 is transmitted from port 1 through the tunable filter 20 toport 2, such that the microwave signal 305 is output as desired.

FIG. 4 is a schematic of the superconducting microwave switch/router 100illustrating reflection as the mode of operation according to one ormore embodiments. In FIG. 4, the tunable filter 20 is tuned such thatthe center angular frequency ω₀ of the microwave signal 305 is greaterthan the cutoff frequency ω_(C) of the tunable filter 20, i.e.,ω₀>ω_(C). In this mode of operation, the tunable filter 20 is configuredto operate in reflection because the frequency of the microwave signal305 is greater than the cutoff frequency of the tunable low-pass filter20. Under this condition, when the microwave signal 305 enters throughport 1, the microwave signal 305 is blocked from passing to port 2because the tunable filter 20 reflects the microwave signal 305, therebynot allowing the microwave signal 305 to pass from port 1 to port 2.

FIG. 5 is a schematic of a superconducting microwave switch/router 100according to one or more embodiments. FIG. 6 is a block diagram of thesuperconducting microwave switch/router 100 in FIG. 5 according to oneor more embodiments. FIG. 6 is an equivalent circuit to FIG. 5 withoutdepicting the internal details of the tunable filter 20. FIGS. 5 and 6are analogous to FIGS. 1 and 2, except that FIGS. 5 and 6 have beenextended to 3 ports instead of 2 ports. It is understood that thesuperconducting microwave switch/router 100 can be extended to N numberof ports as desired according to embodiments.

In the configuration depicted in FIGS. 5 and 6, there are two tunablefilters 20. One tunable filter 20 is connected between port 1 and port2, while the other tunable filter 20 is connected between port 1 andport 3. Each of the tunable filters 20 is formed of one or more unitcells 60 as discussed above. For explanation purposes, the one or morevariable inductors 40 are identified as L₂ in the tunable filter 20connected between ports 1 and 2, while the one or more variableinductors 40 are identified as L₃ in the tunable filter 20 connectedbetween ports 1 and 3. The tunable filters 20 between ports 1 and 2 andports 1 and 3, respectively, are individually controlled such that onecan be in transmission while the other is operating in reflection.

The tunable filter 20 between ports 1 and 2 includes one or more unitcells 60. Each unit cell 60 includes a variable inductor L₂ 40 andcapacitor 50. In each unit cell 60, the variable inductor L₂ 40 isconnected in series with ports 1 and 2, and the capacitor C 50 isconnected to one end of the variable inductor 40 and to ground. Therecan be N number of unit cells 60 repeated and connected together in thetunable filter 20 for a total of N unit cells between ports 1 and 2. Forthe tunable filter 20 between ports 1 and 2, the impedance of each unitcell is Z₂ where

$Z_{2} = \sqrt{\frac{L_{2}}{C}}$and the angular frequency is ω₂ where

$\omega_{2} = {\frac{1}{\sqrt{{CL}_{2}}}.}$

Similarly, the tunable filter 20 connected between ports 1 and 3includes one or more unit cells 60. Each unit cell 60 includes avariable inductor L₃ 40 and capacitor 50. In each unit cell 60, thevariable inductor L₃ 40 is connected in series with ports 1 and 3, andthe capacitor C 50 is connected to one end of the variable inductor L₃40 and to ground. There can be N number of unit cells 60 repeated andconnected together in the tunable filter 20 for a total of N unit cellsbetween ports 1 and 3. For the tunable filter 20 connected between ports1 and 3, the impedance of each unit cell is Z₃

$Z_{3} = \sqrt{\frac{L_{3}}{C}}$where and the angular frequency is ω₃ where

$\omega_{3} = {\frac{1}{\sqrt{{CL}_{3}}}.}$

It should be appreciated that additional ports and tunable filters canbe analogously added as desired.

In FIG. 2, the cutoff frequency of the single tunable filter 20 wasdesignated as ω_(C) above. Because more than one tunable filter 20 isprovided in FIGS. 5 and 6, the tunable filter 20 connected between ports1 and 2 is designated as cutoff frequency Ω_(C2) while the tunablefilter 20 connected between ports 1 and 3 is designated as cutofffrequency ω_(C3).

For operation of the microwave signal 305 in transmission from/betweenport 1 to port 2 (or vice versa), the tunable filter 20 between ports 1and 2 is tuned such that the center angular frequency ω₀ of themicrowave signal 305 is less than the cutoff frequency ω_(C2) of thetunable filter 20 between ports 1 and 2, while the tunable filter 20between ports 1 and 3 is tuned such that the center angular frequency ω₀of the microwave signal 305 is much greater than the cutoff frequencyω_(C3) between ports 1 and 3: ω_(C3)<<ω₀<ω_(C2). In this mode ofoperation, the tunable filter 20 between ports 1 and 2 is configured tooperate in transmission because the microwave signal 305 (ω₀) is lessthan the cutoff frequency ω_(C2), and therefore, the microwave signal305 is transmitted from port 1 through the tunable filter 20 to port 2,such that the microwave signal 305 is output as desired. Concurrently,the tunable filter 20 connected between ports 1 and 3 is configured tooperate in reflection because the microwave signal 305 (ω₀) is greaterthan the cutoff frequency (ω_(C3)), and therefore, the microwave signal305 is blocked from passing between ports 1 and 3. Additional conditionsfor transmission from port 1 to port 2 (or vice versa) include Z₂ ≃Z₀for impedance matching. Additional conditions for reflectionfrom/between ports 1 and 3 include Z₃>>Z₀.

On the other hand, for operation of the microwave signal 305 intransmission from/between port 1 to port 3 (or vice versa), the tunablefilter 20 between ports 1 and 3 is tuned such that the center angularfrequency ω₀ of the microwave signal 305 is less than the cutofffrequency ω_(C3) of the tunable filter 20 between ports 1 and 3, whilethe tunable filter 20 between ports 1 and 2 is tuned such that thecenter angular frequency ω₀ of the microwave signal 305 is much greaterthan the cutoff frequency ω_(C2) between ports 1 and 2:ω_(C2)<<ω₀<ω_(C3). In this mode of operation, the tunable filter 20between ports 1 and 3 is configured to operate in transmission becausethe microwave signal 305 (ω₀) is less than the cutoff frequency ω_(C3),and therefore, the microwave signal 305 is transmitted from port 1through the tunable filter 20 to port 3, such that the microwave signal305 is output as desired. Concurrently, the tunable filter 20 connectedbetween ports 1 and 2 is configured to operate in reflection because themicrowave signal 305 (ω₀) is greater than the cutoff frequency (ω_(C2)),and therefore, the microwave signal 305 is blocked from passing betweenports 1 and 2 in this example. Additional conditions for transmissionfrom port 1 to port 3 (or vice versa) include Z₃ ≃Z₀ for impedancematching. Additional conditions for reflection from/between ports 1 and2 include Z₂>>Z₀.

FIG. 7 is a schematic of a superconducting microwave switch/router 100according to one or more embodiments. FIG. 7 is analogous to FIGS. 5 and6, except that FIG. 7 implements the lossless/superconducting microwaveswitch/router 100 utilizing direct current (DC) superconducting quantuminterference devices (SQUIDs). In FIG. 7, each of the variable inductors40 (discussed above) is implemented as (variable) DC-SQUIDs 705 in thetunable filter 20. It is noted that the tunable filters 20 in FIG. 7 areconfigured to operate in transmission and reflection with respect toeach of the tunable filters 20 as discussed above. Also, it isunderstood that the superconducting microwave switch/router 100 can beextended to N number of ports as desired according to embodiments.

In the configuration depicted in FIG. 7, there are two tunable filters20 and three ports 10 depicted although more ports 10 and tunablefilters 20 can be analogously added. One tunable filter 20 is connectedbetween port 1 and port 2, while the other tunable filter 20 isconnected between port 1 and port 3. Each of the tunable filters 20 isformed of one or more unit cells 60 as discussed herein.

For the tunable filter 20 connected between port 1 and port 2, each unitcell 60 includes one or more DC-SQUIDs 705_2. In the unit cell 60, thecapacitor C 50 connects/shunts the one or more DC-SQUIDs 705_2 toground. When more than one DC-SQUID 705_2 is utilized in the unit cell60, the DC-SQUIDs 705_2 are connected together in series. There can be atotal of M DC-SQUIDs 705_2 per unit cell, where M is an integer of 1 ormore. The tunable filter 20 between ports 1 and 2 includes one or moreunit cells 60, such that each unit cells 60 is connected in series withports 1 and 2, and the capacitor C 50 is connected to one end of theDC-SQUID 705_2 and to ground. There can be N number of unit cells 60repeated and connected together in series in the tunable filter 20 for atotal of N unit cells between ports 1 and 2, where N is an integer of 1or more. For the tunable filter 20 between ports 1 and 2, the impedanceof each unit cell is Z₂ where

$Z_{2} = \sqrt{\frac{L_{2}}{C}}$and the angular frequency is ω₂ where

$\omega_{2} = {\frac{1}{\sqrt{{CL}_{2}}}.}$It is noted that each DC-SQUID 705_2 has an inductance and/or is aninductive element designated L₂.

For the tunable filter 20 connected between port 1 and port 3, each unitcell 60 includes one or more DC-SQUIDs 705_3. In the unit cell 60, thecapacitor C 50 connects/shunts the one or more DC-SQUIDs 705_3 toground. When more than one DC-SQUID 705_3 is utilized in the unit cell60, the DC-SQUIDs 705_3 are connected together in series.

There can be a total of M DC-SQUIDs 705_3 per unit cell, where M is aninteger of 1 or more. The tunable filter 20 between ports 1 and 3includes one or more unit cells 60, such that each unit cell 60 isconnected in series with ports 1 and 3, and the capacitor C 50 isconnected to one end of the DC-SQUID 705_3 and to ground. There can be Nnumber of unit cells 60 repeated and connected together in the tunablefilter 20 for a total of N unit cells between ports 1 and 3, where N isan integer of 1 or more. For the tunable filter 20 between ports 1 and3, the impedance of each unit cell is Z₃ where

$Z_{3} = \sqrt{\frac{L_{3}}{C}}$and the angular frequency is ω₃ where

$\omega_{3} = {\frac{1}{\sqrt{{CL}_{3}}}.}$It is noted that each DC-SQUID 705_3 has an inductance and/or is aninductive element designated L₃.

Now, further information regarding DC-SQUIDs is provided below. A SQUID(Superconducting Quantum Interference Device) is a type ofsuperconducting electronic device well-known to those skilled in theart. In particular, the type of SQUID known as a DC-SQUID includes aloop formed of superconducting wire, superconducting thin-film metal orother superconducting material, interrupted by two or more Josephsonjunctions (JJ) 710. The SQUID contains two or more Josephson junctions710 in a current-carrying loop. As is widely understood by those skilledin the art, via the principle of quantum interference of superconductingcurrents, the combined Josephson critical current of the Josephsonjunctions within the SQUID will vary depending on the magnetic fluxthreading the SQUID loop. Likewise, the Josephson inductance exhibitedby the SQUID's Josephson junctions will also vary depending on suchmagnetic flux (which is magnetic flux Φ₂ for each DC-SQUID 705_2 andmagnetic flux Φ₃ for each DC-SQUID 705_3). Furthermore, arrays of SQUIDscan be arranged in an electrical circuit in such a way as to combinetheir inductances. It is specified that the magnetic flux of an in-planeloop represents a well-known and well-defined quantity including themagnetic field within the loop, multiplied by the cosine of the anglethat the field makes with the axis perpendicular to the loop, integratedacross the entire area of the loop. Thus, the SQUID is highly sensitiveto both the magnitude and the direction of the magnetic field in itsvicinity (for example, flux line 730_2 creates the magnetic field tothereby cause magnetic flux Φ₂ for each DC-SQUID 705_2, while flux line730_3 creates the magnetic field to thereby cause magnetic flux Φ₃ foreach DC-SQUID 705_3). The DC-SQUID 705_2 and 705_3 respectivelyexperience the magnetic flux Φ₂, magnetic flux Φ₃ from the respectivemagnetic fields created by flux line 730_2, flux line 730_3 and therebyits Josephson inductance (the Josephson inductance is designated L_(J2)for DC-SQUID 705_2 and L_(J3) for DC-SQUID 705_3) is changed. To oneskilled in the art, this sensitivity to magnetic field enables the SQUIDto be employed as a useful component in an electric circuit, in that thevariation of the SQUID's Josephson inductance causes useful changes inthe circuit's properties. The inductance L₂ and L₃ of the DC-SQUIDs705_2 and 705_3, respectively, corresponds to the Josephson inductanceL_(J2) for DC-SQUID 705_2 and L_(J3) for DC-SQUID 705_3. Toindependently change/control (increase or decrease) the inductance L₂and L₃ of the DC-SQUIDs 705_2 and 705_3, flux lines 730_2 and 730_3 areprovided. The flux lines are can be generally referred to as flux lines730. The flux lines 730_2 and 730_3 independently apply a magnetic‘bias’ field perpendicular to the SQUID loop of the respective DC-SQUIDs705_2 and 705_3, in order to set the ‘working point’ of the SQUID. Theflux line 730_2 has current I₂ that creates a magnetic field to causethe magnetic bias flux Φ₂ to change as desired. Similarly, the flux line730_3 has current I₃ that creates a magnetic field to cause the magneticbias flux Φ₃ to change as desired. Accordingly, the tunable filters 20between ports 1 and 2 and ports 1 and 3, respectively, are individuallycontrolled such that one can be in transmission while the other isoperating in reflection.

The inductance L₂ (per unit cell 60) for the tunable filter 20 betweenports 1 and 2 can be defined as L₂ ≃ML_(J2)+L_(S), where M is the numberof DC-SQUIDS 705_2 in a unit cell, where L_(J2) is the Josephsonjunction inductance of the DC-SQUID, and where L_(S) is the seriesinductance of the transmission lines 30 (wires) of each unit cell. Theinductance L₂ of each unit cell 60 is primarily based on the Josephsonjunction inductance L_(J2). Therefore, Josephson junction inductanceL_(J2) is defined below (without the series inductance L_(S) of thetransmission line 30 (wires)): the Josephson junction inductance

${L_{J\; 2} = \frac{L_{J\; 0}}{{\cos\left( {\pi\;\frac{\Phi_{2}}{\Phi_{0}}} \right)}}},$where L_(J0)=Φ₀/4πI₀, where I₀ is the critical current of each Josephsonjunction 710, wherein Φ₂ is the magnetic flux bias threading the loop,and where

$\Phi_{0} = \frac{h}{2\; e}$(superconducting magnetic flux quantum) in which h is Planck's constantand e is the electron charge.

Similarly, the inductance L₃ (per unit cell 60) for the tunable filter20 between ports 1 and 3 can be defined as L₃ ≃ML_(J3)+L_(S), where M isthe number of DC-SQUIDS 705_3 in a unit cell, where L_(J3) is theJosephson junction inductance, and where L_(S) is the series inductanceof the transmission line 30 (wires) of each unit cell. The inductance L₃of each unit cell 60 is primarily based on the Josephson junctioninductance L_(J3). Therefore, Josephson junction inductance L_(J3) isdefined below (without the series inductance L_(S) of the transmissionline 30 (wires)): the Josephson junction inductance

${L_{J\; 3} = \frac{L_{J\; 0}}{{\cos\left( {\pi\;\frac{\Phi_{3}}{\Phi_{0}}} \right)}}},$where L_(J0)=Φ₀/4πI₀, where I₀ is the critical current of the (two)Josephson junctions 710, where Φ₃ is the magnetic flux bias threadingthe loop, and where

$\Phi_{0} = \frac{h}{2\; e}$(superconducting magnetic flux quantum) in which h is Planck's constantand e is the electron charge. In this analysis, the experimenters assumethat the DC-SQUIDs have small loops and that the self-inductance of theDC-SQUID loop is negligible compared to the Josephson inductance of theDC-SQUID.

It is noted that the inductance L₂ is the inductance of one unit cell 60out of N unit cells (N≧1) connected in series with the transmission linein the tunable filter 20 between ports 1 and 2, and likewise theinductance L₃ is the inductance of one unit cell 60 out of N unit cells(N≧1) connected in series with the transmission line in the tunablefilter 20 between ports 1 and 3.

It should be understood by one skilled in the art that the tunablefilter design discussed herein is not limited to identical unit cellswith respect to the inductive and capacitive elements in each unit cell.The identical unit cell picture is mainly presented here for simplicityand ease of understanding. In fact, varying the unit cells based on themicrowave filter theory can be advantageous and yield a betterperformance in terms of the maximum amplitude of ripples in the filterresponse, the filter flatness, the filter bandwidth, the amount ofreflection in-band and out-of-band, the amount of attenuation in thestopping band, etc. Accordingly, it should be appreciated that the unitcells may or may not be identical in one or more embodiments to employany or more of the advantages discussed above.

As should be recognized, the superconducting microwave switch/router 100can have one input port and N output ports in one configuration, and/orhave one output port and N input ports in another configuration. Allports 10 of the device have the same characteristic impedance Z₀. Eachinput-output pair is connected through a tunable low-pass filter whosecut-off frequency can be tuned in-situ using applied magnetic flux. Thetunable low-pass filter 20 can be implemented using a ladder ofinductive elements (DC-SQUIDs) and capacitive elements (lumped-elementcapacitors).

By controlling the DC currents I₂ and I₃ respectively applied to theflux lines 730_2 and 730_3, one can independently set the magnetic biasfluxes Φ₂ and Φ₃ which determine inductance L₂ and L₃ in each chain.This in turn tunes the cutoff angular frequencies ω_(C2), ω_(C3) of thetwo tunable filters 20 relative to ω₀ (of the microwave signal 305),such that one path (between ports 1 and 2) is in transmission while theother path (between ports 1 and 3) is in reflection, or vice versa.

To operate in reflection (i.e., block the microwave signal 305) foreither tunable filter 20 (between ports 1 and 2 or between ports 3 and4), one increases the DC currents I₂, I₃ to increase the magnetic biasflux Φ₂, Φ₃ (within 1 period of the cosine), which then increases theinductance L₂, L₃, thereby decreasing the cutoff angular frequencyω_(C2), ω_(C3). Conversely, to operate in transmission (i.e., pass themicrowave signal 305) for either tunable filter 20 (between ports 1 and2 or between ports 3 and 4), one decreases the DC currents I₂, I₃ todecrease the magnetic bias flux Φ₂, Φ₃ (within 1 period of the cosine),which then decreases the inductance L₂, L₃, thereby increasing thecutoff angular frequency ω_(C2), ω_(C3).

The DC-SQUIDs 705, capacitors 50 (with the exception of the dielectricmaterial in the capacitors), flux lines 730, transmission lines 30, andJosephson junctions 710 are made of superconducting material. Again,examples of superconducting materials (at low temperatures, such asabout 10-100 millikelvin (mK), or about 4 K) include niobium, aluminum,tantalum, etc. A Josephson junction is a nonlinear element formed of twosuperconducting metals sandwiching a thin insulator such as, forexample, aluminum oxide, niobium oxide, etc.

FIG. 8 is a schematic of a superconducting microwave switch/router 100according to one or more embodiments. FIG. 8 is analogous to FIGS. 1-7,except for in this implementation, the tunable filters 20 are tunablehigh-pass filters. By having high-pass filters as the tunable filters20, the inductive elements 40, 705 are interchanged with the capacitiveelements 50. Accordingly, the capacitive elements 50 are in seriesbetween port 1 and 2 and between port 1 and 3, while the inductiveelements 40, 705 (inductor or DC-SQUID) connects to one end of thecapacitive element 50 and then connects to ground. For transmission fromport 1 to port 2 (or vice versa), the following condition applies:ω_(C2)<ω₀<<ω_(C3). For transmission from port 1 to port 3 (vice versa),the following condition applies ω_(C3)<ω₀<<ω_(C2).

Now turning to FIG. 9, FIG. 9 is a schematic an N-port superconductingmicrowave router 100 according to one or more embodiments. The N-portsuperconducting microwave router 100 is generalized/designed such thatthere can be a connection made between any pair of ports 10 on the flyusing current pulses to the relevant flux lines which in turn flux biasthe relevant filters to their appropriate flux bias points. For example,at the moment (or nearly at the moment) the microwave signal 305 reachesa port 10, the connection can be made between any pair of ports 10 totransmit the microwave signal 305 while all other ports 10 (via theirrespective tunable filter 20) block the microwave signal 305.Accordingly, the microwave signal 305 can be routed between any pair ofports 10 according to the principles discussed herein.

The N-port superconducting microwave router 100 includes port 1, port I,port J, through port N. Each of the port 1-N has its own tunablelow-pass filter 20, such that an individual port 10 connects to atunable filter 20 that connects to a node 905. The features extensivelydescribed above in FIGS. 1-8 apply to FIG. 9 and are not repeated forthe sake of brevity and to avoid obscuring FIG. 9. All of the ports 1-Nare symmetrical and are on the same footing (which is different from thepreviously described superconducting microwave switches/router 100above). Being on the same footing means that the node 905 is a centralconnection that connects all of the ports 1-N, that each port 10 has itsown tunable filter 20, and that each tunable filter 20 has its own fluxline (FL) for tuning its cutoff frequency.

As one example, to route the microwave signal 305 from port N to port I,both tunable filters 20 between port N and node 905 and between port Iand node 905 have to be tuned to be in transmission; concurrently, allremaining tunable filters 20 are tuned to be in reflection. This allowsthe microwave signal 305 to be transmitted from port N to its tunablefilter 20, to the node 905, to tunable filter 20 connected to port I,and then transmitted to port I.

Regarding the node 905, a few technical details are discussed. Ingeneral, node 905 is to be as small as possible and ideally lumped withrespect to the wavelengths used in the device operation for tworeasons: 1) minimize reflections, which can limit the transmission ofthe routed signal, and 2) enable connecting multiple transmission linesto the node 905. Furthermore, the ability to connect multipletransmission lines to a common node 905 can require using high impedance(very narrow) wires, which might in turn require designing the tunablefilters to have a characteristic impedance which matches the impedanceof the connecting lines when the filters are operating in transmission(in order to minimize reflections) in one implementation. Lastly, if thecharacteristic impedance of the tunable filters is different from thecharacteristic impedance of the device ports, certain matching networkscan be designed and integrated between the filters and the device (inorder to allow smooth transmission for the propagating signals).

FIG. 10 is a flow chart 1000 of a method of configuring alossless/superconducting microwave switch/router 100 according to one ormore embodiments. Reference can be made to FIGS. 1-9 discussed herein.

At bock 1005, a plurality of ports 10 are provided. At block 1010,tunable filters 20 are provided and connected to the ports 10, such thateach of the plurality of ports 10 has a corresponding one of the tunablefilters 20.

The tunable filters 20 connect to a node 905 (a conductive connectionpoint). A plurality of flux lines (FL) 730 are provided, such that anindividual one of the plurality of flux lines 730 tunes an individualone of the tunable filters 20 on a one-to-one basis. A plurality ofmagnetic sources (such as flux lines, current carrying wires, tunablemagnets, etc.) are provided, such that an individual one of theplurality of magnetic sources tunes an individual one of the tunablefilters 20 on a one-to-one basis. It should be noted that this pictureof one flux line controlling one tunable filter can be simplistic. Thisis because the DC-SQUID's response/inductance is determined by the totalflux threading its loop, and therefore any change in the current ofother flux lines can alter, in principle, the flux bias experienced bythe DC-SQUID. Of course, the induced flux by the other flux lines dropsconsiderably with the distance between them and the DC-SQUID, thus bykeeping them sufficiently apart the experimenters can significantlyreduce their contribution. Nevertheless, there can be one or morescenarios that in order to tune the flux bias of one filter, one mightapply multiple changes to the currents flowing in nearby flux lines suchthat the currents yield the desired flux bias in the various controlledfilters.

The tunable filters 20 include superconducting material. Examplesuperconducting materials at superconducting temperatures (e.g., 10-100millikelvin (mK), or 4 K) can include niobium, aluminum, tantalum, etc.

The tunable filters 20 can be tunable lossless low-pass filters. Any oneof the plurality of ports 10 (e.g., port 1) is configured to transmit amicrowave signal 305 to any other one of the plurality of ports 10 (port2). The corresponding one of the tunable filters 20 for the any one ofthe plurality of ports 10 and the corresponding one of the tunablefilters 20 for the any other one of the plurality of ports 10 are bothconfigured to be tuned to transmit (i.e., in transmission) the signal305 while all other ones of the tunable filters 20 are configured to betuned to block the signal. Each of the lossless low-pass filtersincludes one or more DC SQUIDS in series with a center conductor of atransmission line and shunted by a capacitor to ground. It should beappreciated that a transmission line, such as a coaxial cable, has acenter conductor and an outer conductor.

FIG. 11 is a flow chart 1100 of a method for configuring alossless/superconducting microwave switch/router 100 according to one ormore embodiments. Reference can be made to FIGS. 1-9 discussed herein.

At block 1105, a plurality of ports 10 are provided. At block 1110,tunable filters 20 are connected to the plurality of ports 10, whereeach of the plurality of ports 10 is associated with one of the tunablefilters 20, where each of the tunable filters 20 includes asuperconducting quantum interference device 705. The tunable filters 20can be low-pass filters. The tunable filters 20 can be high-passfilters.

FIG. 12 is a flow chart 1200 of a method of configuring alossless/superconducting microwave switch/router 100 according to one ormore embodiments. Reference can be made to FIGS. 1-9 discussed herein.

At block 1205, a node 905 is provided a central connection point. Atblock 1210, tunable filters 20 are connected to the node 905, where thetunable filters 20 are configured to be independently tuned to a firststate (i.e., mode of operation for transmission) to transmit a microwavesignal 305 and be independently tuned to a second state (i.e., mode ofoperation for reflection) to block the microwave signal 305, such thatany one of the tunable filters 305 is configured to transmit the signalto any other one of the tunable filters 20 via the node 905.

Any one of the tunable filters 20 and the any other one of the tunablefilters 20 are both configured to be in the first state, while allremaining tunable filters 20 are configured to be in the second state,thereby allowing the microwave signal 305 to be transmitted from the anyone of the tunable filters 20 to the any other one of the tunablefilters 20 via the node 905.

FIG. 13 is a flow chart 1300 of a method of configuring alossless/superconducting microwave switch/router 100 according to one ormore embodiments. Reference can be made to FIGS. 1-9 discussed herein.

At block 1305, a plurality of ports 10 are provided. At block 1310, afirst pair of the plurality of ports 10 has at least one tunable filter20 connected in between, in which the tunable filter 20 is configured totransmit a microwave signal 305. At block 1315, a second pair of theplurality of ports 10 has another tunable filter 20 connected inbetween, in which the another tunable filter 20 is configured to reflectthe microwave signal.

Technical effects and benefits include a lossless/superconductingmicrowave switch/router. Technical benefits further include lowattenuation of transmitted signals <0.05 dB (decibels), fast switching(no resonators) such as in nanoseconds (depending on the mutualinductance between the flux lines and the SQUIDs), and relatively largebandwidth (bw) >280 megahertz (MHz) (which can be significantly enhancedby allowing certain variation in the unit cells). Also, technicalbenefits further include relatively large on/off ratio >20 dB. Thelossless/superconducting microwave switch/router can tolerate relativelylarge powers >−80 dBm (where 0 dBm corresponds to 1 milliwatt) by addingmore SQUIDs and increasing their critical current. Thelossless/superconducting microwave switch/router can be fabricated withNb Josephson junctions to operate at 4K, can be designed for anyfrequency range, and provides a scalable scheme that can be easilyextended to 1 input-N outputs (or vice versa).

The term “about” and variations thereof are intended to include thedegree of error associated with measurement of the particular quantitybased upon the equipment available at the time of filing theapplication. For example, “about” can include a range of ±8% or 5%, or2% of a given value.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams can represent a module, segment, or portionof instructions, which includes one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block can occur out of theorder noted in the figures. For example, two blocks shown in successioncan, in fact, be executed substantially concurrently, or the blocks cansometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments discussed herein. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdiscussed herein.

What is claimed is:
 1. A lossless microwave switch comprising: aplurality of ports; and tunable filters, wherein each of the pluralityof ports is operatively coupled to a corresponding one of the tunablefilters, wherein the tunable filters are lossless low-pass filters. 2.The lossless microwave switch of claim 1, wherein the tunable filtersconnect to a node.
 3. The lossless microwave switch of claim 1, furthercomprising a plurality of flux lines; wherein an individual one of theplurality of flux lines tunes an individual one of the tunable filterson a one-to-one basis; or wherein one or more flux lines of theplurality of flux lines contributes to tuning the individual one of thetunable filters.
 4. The lossless microwave switch of claim 1, furthercomprising a plurality of magnetic sources, such that an individual oneof the plurality of magnetic sources tunes an individual one of thetunable filters on a one-to-one basis.
 5. The lossless microwave switchof claim 1, wherein the tunable filters comprise superconductingmaterial.
 6. The lossless microwave switch of claim 1, wherein the eachof the lossless low-pass filters includes one or more DC SQUIDS inseries with a center conductor of a transmission line and shunted by acapacitor to ground.
 7. The lossless microwave switch of claim 1,wherein the tunable filters are lossless high-pass filters.
 8. Thelossless microwave switch of claim 7, wherein each of the losslesshigh-pass filters include a capacitor shunted by one or more DC SQUIDS.9. The lossless microwave switch of claim 1, wherein any one of theplurality of ports is configured to transmit a signal to any other oneof the plurality of ports.
 10. The lossless microwave switch of claim 9,wherein the corresponding one of the tunable filters for any one of theplurality of ports and the corresponding one of the tunable filters forthe any other one of the plurality of ports are both configured to betuned to transmit the signal while all other ones of the tunable filtersare configured to be tuned to block the signal.
 11. A method ofconfiguring a lossless microwave switch, the method comprising:providing a plurality of ports; and providing tunable filters, such thateach of the plurality of ports is operatively coupled to a correspondingone of the tunable filters, wherein the tunable filters are losslesslow-pass filters.
 12. The method of claim 11, wherein the tunablefilters connect to a node.
 13. The method of claim 11, furthercomprising providing a plurality of flux lines; wherein an individualone of the plurality of flux lines tunes an individual one of thetunable filters on a one-to-one basis; or wherein one or more flux linesof the plurality of flux lines contributes to tuning the individual oneof the tunable filters.
 14. The method of claim 11, further comprisingproviding a plurality of magnetic sources, such that an individual oneof the plurality of magnetic sources tunes an individual one of thetunable filters on a one-to-one basis.
 15. The method of claim 11,wherein the tunable filters comprise superconducting material.
 16. Themethod of claim 11, wherein any one of the plurality of ports isconfigured to transmit a signal to any other one of the plurality ofports.
 17. The method of claim 16, wherein the corresponding one of thetunable filters for the any one of the plurality of ports and thecorresponding one of the tunable filters for the any other one of theplurality of ports are both configured to be tuned to transmit thesignal while all other ones of the tunable filters are configured to betuned to block the signal.
 18. A lossless microwave switch comprising: aplurality of ports; and tunable filters, wherein each of the pluralityof ports is associated with one of the tunable filters, wherein each ofthe tunable filters includes a superconducting quantum interferencedevice.
 19. The lossless microwave switch of claim 18, wherein thetunable filters are low-pass filters.
 20. The lossless microwave switchof claim 18, wherein the tunable filters are high pass filters.
 21. Alossless microwave switch comprising: a node; and tunable filtersconnected to the node, wherein the tunable filters are configured to beindependently tuned to a first state to transmit a signal and beindependently tuned to a second state to block the signal, such that anyone of the tunable filters is configured to transmit the signal to anyother one of the tunable filters via the node.
 22. The losslessmicrowave switch of claim 21, wherein the any one of the tunable filtersand the any other one of the tunable filters are both configured to bein the first state, while all remaining ones of the tunable filters areconfigured to be in the second state, thereby allowing the signal to betransmitted from the any one of the tunable filters to the any other oneof the tunable filters via the node.